This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.11-319010 filed on Nov. 10,1999, No. 2000-112172 filed on Apr. 13, 2000 and No. 2000-230299 filed on Jul. 31, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel injection valve applicable to common rail fuel injection systems for internal combustion engines, in particular, to the construction of the fuel injection valve.
2. Description of Related Art
As a fuel supply system for internal combustion engines, a common rail fuel injection system, in which fuel pressurized by a high pressure supply pump is accumulated in a common rail and injected to each engine cylinder at a given timing, is well known. The common rail fuel injection system has merits that injection timing and injection amount are accurately controlled. As a fuel injection valve of the common rail fuel injection system, as disclosed in U.S. Pat. No. 5,819,710, known is a fuel injection valve in which a valve opening and closing force for moving a nozzle needle to open and close an injection hole is given by hydraulic pressure in a control chamber. A three ways valve controls hydraulic pressure in the control chamber. The three ways valve has a valve body to be driven by a piezoelectric actuator. The control chamber is communicated selectively to a low pressure passage or a high pressure passage according to a position where the valve body is seated. When the valve body is driven so as to open the low pressure passage and close the high pressure passage, the control chamber is communicated to the low pressure chamber so that hydraulic pressure in the control chamber may be decreased and the nozzle needle may be moved to open the injection hole through which fuel is injected.
The conventional injection valve mentioned above is provided with an orifice between the three ways valve and the control chambers. The orifice serves to adjust a valve opening speed of the nozzle needle and a valve closing speed thereof. Since, at both of a valve opening time and a valve closing time, fuel passes through the same orifice, decreasing and increasing speeds of hydraulic pressure in the control chamber can""t be independently controlled. It is preferable, as an injection characteristic, that the decreasing speed of hydraulic pressure in the control chamber is relatively slow so as to move slowly the nozzle needle at the valve opening time and the increasing speed of hydraulic pressure is relatively fast so as to move rapidly the nozzle needle at the valve closing time. However, in the conventional fuel injection valve, as a diameter of the orifice becomes larger, both of increasing and decreasing speeds of hydraulic pressure become higher. On the contrary, a diameter of the orifice becomes smaller, both of increasing and decreasing speeds of hydraulic pressure become lower. Therefore, the fuel injection valve having slow valve opening and rapid valve closing characteristics is desired.
An object of the present invention is to provide a fuel injection device in which a pressure decreasing speed in a control chamber is relatively low and a pressure increasing speed in the control chamber pressure is relatively high so that the nozzle needle may be moved slowly at a valve opening time and rapidly at a valve closing time.
To achieve the object, the fuel injection valve is composed of a nozzle needle for opening and closing an injection hole, a control chamber for urging the nozzle needle in a direction of closing the injection hole when hydraulic pressure is supplied thereto, low pressure and high pressure conduits, a three ways valve having a valve chamber, a valve body and first, second and third ports, a first conduit communicating the first port to the low pressure conduit, a second conduit communicating the second port to the high pressure conduit, a third conduit communicating the third port to the control chamber and a fourth conduit communicating the control chamber to the high pressure conduit. The third and fourth Conduits are provided with a main orifice and a sub orifice, respectively.
With the construction mentioned above, the control chamber may be communicated to the low pressure conduit via the third conduit, the valve chamber and the first conduit, when the valve body opens the first port and closes the second port, and be communicated to the high pressure conduit via the third conduit, the valve chamber and the second conduit, when the valve body closes the first port and opens the second port.
Further, at the valve opening time, when the valve body is driven to open the first port and the pressure of the valve chamber is reduced, pressure in the control chamber is reduced. On the other hand, high pressure is still applied to the control chamber since the control chamber is always communicated to the high pressure passage via the fourth conduit with the sub orifice. Therefore, a pressure decreasing speed in the control chamber is relatively low and the movement of the nozzle needle for opening the injection hole becomes relatively slow.
Then, at the valve closing time, when the valve body is driven to close the first port and pressure of the valve chamber is increased, pressure in the control chamber is increased. At the same time, high pressure is directly applied to the control chamber from the high pressure conduit via the fourth conduit with the sub orifice. Therefore, a pressure increasing speed in the control chamber is relatively high and the movement of the nozzle needle for closing the injection hole becomes relatively fast.
When a diameter of the sub orifice is small relatively to a diameter of the main orifice, pressure in the control chamber can""t be increased with a sufficiently high speed. On the other hand, when the diameter of the sub orifice is large relatively to that of the main orifice, a minimum valve opening pressure of the nozzle needle becomes too high. Accordingly, it is preferable to secure both of sharp valve closing characteristic and low minimum valve opening pressure that a diameter ratio of the sub orifice to the main orifice (a diameter of the sub orifice/ a diameter of the main orifice) falls within a range from 0.5 to 1.0 when the minimum valve opening pressure of the nozzle needle is set to not larger than 20 Mpa.
Further, when the minimum valve opening pressure of the nozzle needle is set to not larger than 30 Mpa, the diameter ratio of the sub orifice to the main orifice may fall within a range from 0.6 to 1.2 to realize the sharp valve closing characteristic and the available low minimum valve opening pressure.
Preferably, the fuel injection valve has a spring chamber communicated to the high pressure conduit. A spring for urging the nozzle needle in a valve closing direction is housed in the spring chamber. A head of the nozzle needle is also housed in the spring chamber and is provided with the control chamber and the fourth conduit with the sub orifice connecting the spring chamber and the control chamber. The third conduit with the main orifice extends from the control chamber to the second port through space of the spring chamber but without communicating to the spring chamber.
According to the construction mentioned above, as the control chamber is formed inside the head of the nozzle needle, a volume of the control chamber is relatively small so that good controllability may be secured. Further, the third conduit may be constituted and easily manufactured by, for example, a pipe member and the main orifice also may be easily manufactured. Furthermore, as the fourth conduit with the sub orifice is formed in the head and their constructions are simple, manufacturing of the same is very easy.
It is preferable to have a nozzle lift stopper with which an upper end of the nozzle needle may come in contact and by which a movement of the nozzle needle may be stopped for restricting a lift amount thereof. Unless the fuel injection valve is provided with the stopper, the lift amount becomes unnecessarily large at the valve closing time so that a moving distance of the nozzle needle becomes longer, resulting in taking a longer time for closing the injection hole. The fuel injection valve having the stopper mentioned above has a good valve closing response characteristic.
Preferably; the third conduit having the main orifice is opened to a surface of the stopper portion facing the control chamber so that the nozzle needle may close the third conduit when the upper end surface of the nozzle needle is in contact with the stopper. With the construction mentioned above, at the valve closing time, high pressure of fuel flowing into the third conduit via the valve chamber is applied to the upper end surface of the nozzle needle. Accordingly, the nozzle needle may start the valve closing operation immediately without spending a time during which high pressure fuel enters into a clearance between the upper end surface of the nozzle needle and the stopper surface from an outer periphery thereof.
The main orifice is, preferably, formed in the third conduit at an opening end to the control chamber. To improve a valve closing response characteristic, it is better that an area where the nozzle needle and the stopper are in contact with each other is smaller. As the main orifice is formed at the opening end of the control chamber, an area of the stopper may be smaller so that the area where the nozzle needle and the stopper are in contact with each other may become smaller.
At least the second and fourth conduits may be formed in a block shaped conduit-forming member disposed between the valve chamber and the control chamber whose one end surface constitutes a part of a wall of the valve chamber and whose another end surface constitutes a part of a wall of the control chamber. The conduit-forming member is provided with the high pressure conduit penetrating from the one end surface thereof to the another end surface thereof, a branch conduit extending from the high pressure conduit, the second conduit extending from the branch conduit to the valve chamber, and the fourth conduit having the sub orifice extending from the branch conduit to the control chamber. In this case, respective connections of the branch conduit to the high pressure conduit, to the second conduit and to the fourth conduit are made, preferably, at an angle more than a near right angle so that the respective connections of the branch conduit may be prevented from being harmed by high pressure fuel. Further, as the conduit-forming member is a single element, the fuel injection valve may be composed of a less number of parts and component at a lower cost.
The branch conduit may be constituted by a hole extending horizontally from the high pressure conduit toward an inside of the conduit-forming member or a groove extending from the high pressure conduit on at least one of end surfaces of the conduit-forming member.
The groove may be provided on each of opposite end surfaces of the conduit-forming member. The second conduit is connected to one of the grooves and the fourth conduit having the sub orifice is connected to the other of the grooves. In this case, if the grooves extend in opposite directions from the high pressure conduit so as to put the second and fourth conduits therebetween, the second and fourth conduits may be easily manufactured without interfering with each other.